Automatic amplification control



June 9, 1942.

G. M. BROWN AUTOMATIC AMPLIFICATION CONTROL Original Filed June 13, 1939CARRIER WAVE SOURCE Inventor"; George M. Brown,

His Attorney Patented June 9, 1942 aurorra'rrc amurrcarrou common 7George M. Brown, Bcotia, N. Y., ullgnor to General Electric Company, acorporation of New York Original application June 13,, 1939, .Serial No.278,854. Divided and this application July 23, 1941, Serial No. 403,658I r g Claims. (01. na -171.5)

This invention relates to electron discharge amplifier circuits.

This application is a division of my former application,Serial No.278,854, filed June 13, 1939, and assigned to the same assigneeas thepresent application.

It has been a problem inthe art of communication by modulated carrierwaves to obtain a in order to use the carrier wave most efficiently. Itis desirable to amplify signal waves with which a carrier is to bemodulated in such a way that the average modulation depth approaches 100per cent as nearly as possible while retaining the fidelity of thesignal waves. The design of my I maximum amount of modulation at alltimes.

amplifier is such that signal waves are amplified linearly up to somearbitrary amount which modulates the carrier wave at an arbitrary depth,such as 90, per cent, while for signal waves of greater level than thisarbitrary value the amplification is reduced so that the output of myamplifier increases very slowly above this arbitrary value. It isaccordingly an' object of my invention to provide an improved andsimplified audio amplifier of conventional construction which is adaptedto supply amplified signal energy to any utilization circuit and at thesame time controls the amount of amplification in response to therelation of the amplified wave to uni-arbitrary level.

In certain types of amplifiers, such as those used with transmitters offrequency modulated waves, or phonograph recording apparatus and a thelike, it is desirable to provide means responsive to peaks of the signalwaves of either polarity to control the amplification of the amplifier.In the case of frequency modulated waves the available band width of thereceiver is definitely limitedand the frequency modulated wavestransmitted must not exceed this band width in their frequency changes.In phonographic recording the available groove width must not beexceeded on either side. Many communication systems have similarlimitations. It is therefore an additional object of my invention toprovide improved amplifying apparatus continuously adjusted in amount ofamplification in response to the level of peaks of alternating voltageof either polarity.

The features of my invention which I believe to be novel'are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof may best be understood byreference to the following description taken in connection with theaccompanying drawing in which the single figure is a circuit diagramillustrating;

schematically .an audio amplifier embodying my invention.

Referring. to the figure. a suitable source of.

audio frequency waves is'represented by an input transformer iii, acrossthe secondary of which a voltage dividing resistance ii is connected..

The primary of the transformer l0 may be suitably supplied with audiofrequency waves from .a microphone or a phonograph pickup, or the like.One end of the resistance II is connected to ground and is alsoconnected through a oath-- ode bias-resistor i2 and a bypass condenserl3 to the cathode of a pentagrid tube It. The movable contact of theresistance is connected to the first grid of tube il,.which serves toamplify voice waves applied through transformer i0 The anode of the tubei4 is eonnected'through a resistor Ii and a resistor it to a sui ablesource of operating potential; grids of the tube H are screen grids andare connected through a resistor IT to a point between resistors II andIS in order to provide the screen grids with'the proper biasingpotential. These two screen grids are connected by a condenser i8 to thecathode of tube it. Another condenser is connects the point betweenresistors l5 and IS with the cathode of tube H. The fifth gridof tube Itis connected directly to the cathode and acts as a suppressor grid.

Voice waves applied to the first or control grid of the tube I throughthe transformer i0 produce corresponding variatio nsin anode potentialof the tube i4 due to the variations in voltage drop through resistorl5. These variations in voltage are coupled by a condenser 20 to thecontrol grid of a triode tube 38. The cathode of the tube 38 isconnected to ground through.

cathode biasing resistor 22. A gridresistor-fl I is connected from thegrid oftube 38 to ground to provide proper bias potential for the grid.

\Resistor 22 is shunted by a bypassing condenser through airesistor 26to a suitable sourceof operating potential. a J.

The grid of tube 38, which is supplied with voltage variations bycondenser 20, causes voltage variations on the anode of tube 38 due to vthe voltage drop through the resistor 26. These anode voltage variationsare coupled through'a condenser 21 to the primary of a transformer 28.

The other end of the transformer primary'is connected to the cathode oftube. 38. The ends of the secondary of transformer 28 are connected Thesecond and fourth The anode of .the tube 38 is connected resistor 46.

respectively to the grids of a pair of tubes 29 and 30 arranged inpush-pull relation. The secondary of the transformer 28 has a center tapconnection which is grounded and is also connected through a resistor 3|shunted by a bypass condenser 32 to the cathodes of the tubes 29 and 36.The anodes of the tubes 29 and 36 are con-- nected respectively to theends of the primary of a transformer 33. of the primary of transformer33 is connected to a suitable source of anode potential. The secondaryof the transformer 33 is adapted to supply amplified audio waves to anysuitable device. This device may, for example, be the modcarrier wavesource 63 impresses a carrier wave upon the device 6| to be amplifiedtherethrough by an amount governed by the operating potenl 2,2sq,

43, even such minimum feedback produces condenser 45, one half of tube43, and resistance 36 to the third grid of tube. l4, but alternatingpotential of opposite phase is fed back simultane- A center tapconnection ously from tube 30 through condenser 44, the other half oftube 43, andresistance 36 to the third grid of tube I4. These twoalternating potential of opposite phase produce substantially completeneutralization of each other on the third grid of tube l4, and thus bothregenerative and degenerative effects in the grid control connectionsare avoided.

The proper adjustment of my amplifier for use in a radio transmitter issuch that the outtial supplied through transformer 33. The amplifledcarrier wave inthe tuned circuit 62 is radiated from an antenna 64.

A pair of voltage dividing resistances 33 and 40 are connected across asuitable source of direct current. The sliding contact of the resistance39 is connected through a conductive impedance 4|, which is shown forconvenience .as a pure resistance, to the cathode of a diode element ofa tube 43. Similarly the sliding contact of the resistance 40 isconnected through a conductive impedance 42, also shown as a resistor,to the cathode of a second diode element of the tube 43. The cathode ofthe flrstvdiode is connected through a condenser 44 to the anode of tube30. The cathode of the other diode is connected through a similarcoupling condenser 45 to the anode of; tube 23 The anodes of these twodiodes are connected together and to ground through'a resistor 46. Theseanodes are also connected through resistor 36 to the third grid of tubel4, which is coupled by the condenser 31 to ground.

In operation, under the condition when no signal is being amplified, thecathodes of the tube 43 are maintained at positive potentials withrespect to ground by amounts equal to the voltage from the slidingcontacts of the respective resistances 39 and 46 to ground. The anodesof tube 43 are maintained at ground potential ,by

when signals are amplified by the arrangement, the voltage variations'ofthe anodes of tubes2a and 36 are impressed on the cathodes of tube 43.

These voltage variations due to signals make the cathodes of tube 43less positive during certain portions of the signal wave. When thesecathodes become negative due to a signal of greater level than thatnecessary to overcome the respective positive bias voltage produced byresistances 33 and 40, the tube 43 passes current and the anodes of tube43 become negative with respect to ground. This produces a similarnegative voltage on the third {grid of tube l4 and reduces theamplification thereof.

Although there i little capacity between the .anode and cathode of adiode, so that feedback is a minimum through such capacity in the tubeput of transformer33 rises linearly in accordance with the input totransformer l0 up to a critical value at'which the output from thetransformer 33 is suflicient; to modulate the carrier wave with apredetermined depth, such, for example, as 90 per cent. Any othersuitable depth may 'be chosen as desired. The operation of my amplifierthen prevents any great rise in output from transformer 33 above thisvalue and accordingly reduces tremendously the possibility ofovermodulation. With an amplifier of this type there can be noappreciable distortion of. the modulated carrier wave with correspondingradiation at undesired frequencies, since over-modulation may becompletely prevented except for momentary transients.

It has been found that some music and most speech, when translated intoelectric wave energy, 7

' mum carrier amplitudes and that the smaller peaks of opposite polarityproduce the negative modulation peaks of the carrier. The provision ofsuch operation allows more energy to be radiated for a given signalwave, assuming maximum permissible modulation, with a correspond-' ingincrease in efliciency. It is desirable, therefore, to secure suchoperation, to pole the output from transformer 33 to insure that thepeaks of higher polarity produce positive modulation peaks. It is, ofcourse, first desirable to pole the input to transformer ID, from themicrophone or the like, so that the reduction of amplification shall beinitiated in response to the lower peaks of the signal wave.

It should be noted particularly that tube 23 amplifies signal voltageswings of one polarity and tube 30 those of the other.

Hence, if the signal peaks of either polarity increase more thanpredetermined amounts'Fthe tube 43 passes current in the correspondingdiode element and the amplification of tube I4 is reduced. It should Itubes 29 and 30 for the particular type of signal being amplified.

While I have shown a particular embodiment of my invention, it will, orcourse, be understood that I do not wish to be limited thereto, sincedifierent modifications may be made both in the circuit arrangement andinstrumentalities employed, and-I aim by the appended'claims to coverany such modifications as fall within the true spirit and scope of myinvention. What I claim as new and desire to secure by 'LettersPatentvof the United States is: e

1. In combination, an amplifier having a balanced output circuit, a pairof rectifiers, said rectifiers having a direct current load resistance,separate means to bias each rectifier to a normally non-conductivecondition, means to supply alternating potential from the' respectiveopposite sides of said output circuit to a corresponding one of saidrectifiers, whereby each rectifier becomes conductive when itsrespective bias is overcome by said alternating potential and directcurrent tends to flow in the load resistance, and means to control theamplification of'said amplifier in response tothe potential on saidresistance, the bias supplied to the diiIerent rectifiers by saidbiasing means. being proportioned in accordance with the magnitude ofalrectifier to a normally non-conductive condition, means to supplyalternating potential from the respective opposite sides of saidbalanced circuit to a corresponding one of said rectifiers, whereby eachrectifier becomes conductive when its respective bias is overcome bysaid altemating potential and direct potential is impressed on saidpoint, and means to control the amplification of said amplifier inresponse to the potential on said point, the bias supplied to thedifi'erent rectifiers by said biasing means being proportioned inaccordance with the magnitude of alternating voltage on the respectivesides of said balanced circuit.

4. In an electron discharge amplifying device including a balancedcircuit, a pair or unidirectional conducting circuits, separate means ineach of said unidirectional circuits to bias said circuits to a normallynon-conductive condition, means to supply alternating potential from therespective opposite sides of said balanced circuit to a correspondingone of said unidirecternating voltage on the respective sides of theoutput circuit. 7

2. Modulating means for modulating the intensity of a carrier wave inaccordance with a signal, said means comprising a signal amplifierhaving separate rectifiers for rectifying positive and negative halfcycles of said signal, a separate bias potential source tending tomaintain each of said rectifiers in non-conducting condition, wherebycurrent fiows in the respective rectifiers only when said positive andnegative .half cycles exceed the corresponding bias potentials, andmeans to reduce the amplification of said amplifier in response tocurrent flowing in either of said rectifiers, said positive half cyclesbeingof greater intensity than said negative half cycles and said'modulating means being arranged to decrease the intensity of saidcarrier in response to negative halI cycles 01' signal, said biaspotential corresponding to said 7 negative half cycles being adjustedsubstantially to prevent decrease oisaid carrier wave to zero intensity,and said bias potential corresponding to said positive half cycles beingadjusted to to the direct potential on said point prevent increase ofsaid carrier above a predetermined intensity.

3. In an electron discharge amplifying device including a balancedcircuit, a pair of rectifiers i connected separately tov impress adirect potential ona point, separate means to bias each tional circuits,whereby each of said unidirectional circuits becomes conductive when itsrespectivebias is overcome by plification at said amplifying deviceinresponse I produced by either of said unidirectional conducting cir-0111128. I 1 I Q 5. In a system having a carrier 'wave source,

a signal source, and amplifying means for modulating the intensity-ofthe carrier wave from said carrier wave source in accordance-with thesignal from said signal source, the combination ofv a pair ofrectiflers, separate means to bias each rectifier to a normallynon-conductive condi-- tion, said biasing means being separatelyadjustable whereby conduction occurs in each rectifier 'at a differentpotential, means to supply said amplifying alternating potential ofopposite phase irom said amplifying means to the other ofsaidrectifiers, whereby each or said rectifiers becomes conductive whenits respective bias is overcome by the correspondingalternating-potential and means to control the amplification of saidamplifying means in response to the conduction ofcurrent through eitherof said rectifiers, whereby either increase or decrease of intensity ofsaid carrier wave may belimited to predetermined dinerently adjustablelimits.

enema M. naown.

said alternating potential and tends to maintain a point at a directpotential, and-means to control the am-

